Method and System for Spread Spectrum Signal Acquisition

1. A method for the acquisition of a direct-sequence spread spectrum signal, which is transmitted on a carrier frequency, and which is modulated with a code signal of length equal to Nc chips, for determining a code delay of said spread spectrum signal and a Doppler shift with respect to said carrier frequency, said determination being performed on a discrete two-dimensional space of M possible code delays and F possible frequency shifts, the method comprising: receiving and sampling said spread spectrum signal to obtain a sampled spread spectrum signal; performing a despreading operation of said sampled spread spectrum signal with a local replica signal of said code signal, by performing said despreading for a plurality of possible code delays between said sampled signal and said local replica signal, wherein the despreading includes despreading using consecutive long segments of the sampled signals, the long segments being of a length N, where N is a submultiple integer of a sample number denoted by M of the local replica signal, a number Mn of the despreaded segments of the sampled signals being given by Mn=M/N; by performing a step of computing a Fourier transform, wherein said frequency search is a parallel frequency search performed on a result of said despreading operation, and said Fourier transform is a fractional Fourier transform performed on said result; and performing, using each of said Mn segments of the local replica signal, the despreading for N possible successive delays between said local replica signal and said sampled signal.

2. The method according to claim 1 , wherein the despreading step provides at output a plurality of despreaded vectors, said plurality of despreaded vectors comprising Mn despreaded vectors for each code delay.

3. The method according to claim 2 , wherein said step of computing a Fourier transform comprises a step of computing the Fourier transform for each of said despreaded vectors of said plurality of despreaded vectors, for obtaining a plurality of respective transformed vectors in a discrete frequency domain.

4. The method according to claim 1 , wherein the despreading step comprises a step of multiplying, sample by sample, signal segments by respective replica segments, providing respective product vectors, and wherein said step of computing the Fourier transform is performed on each of said product vectors.

5. The method according to claim 1 , further comprising: coherently accumulating, into cumulative vectors, despreaded vectors associated to a same code delay, and providing N cumulative vectors, each associated to a respective delay.

6. The method according to claim 5 , wherein the coherently accumulating step is performed after said step of computing the Fourier transform and in such a way as to coherently accumulate said transformed vectors in the frequency domain.

7. The method according to claim 6 , wherein said coherently accumulating step comprises a step of canceling a phase shift of a partial result of said accumulation, before adding a new contribution to said partial result.

8. The method according to claim 1 , wherein said step of receiving and sampling comprises a step of storing said sampled signal into a delay line.

9. The method according to claim 8 , wherein the delay line is a delay line with N taps.

10. The method according to claim 9 , wherein said local replica signal is stored in a matrix data structure comprising Mn N-long vectors, where Mn corresponds to a number of columns in the matrix data structure and N corresponds to a number of rows of the matrix data structure, each for storing a segment of N samples of said code, and wherein successive sample segments of said code are sequentially stored into successive vectors of said matrix, the despreading step comprising a step of multiplying N times one vector of said matrix by the content of said delay line, said delay line being respectively updated with samples of said received signal.

11. The method according to claim 10 , further comprising a step of searching a maximum, after performing at least one scan, by means of said multiplication, on all vectors of said matrix, and further comprising a step of ascertaining the acquisition by means of comparing said maximum with a threshold.

12. The method according to claim 11 , further comprising: estimating, starting with a result of said step of computing the Fourier transform, a noise power; and adaptively varying said threshold, based on said estimated noise power.

13. An acquisition system configured to acquire a direct-sequence spread spectrum signal transmitted on a carrier frequency and modulated with a code signal of length equal to Nc chips, to determine a code delay of said spread spectrum signal and a Doppler shift with respect to said carrier frequency, said determination being performed on a discrete two-dimensional space of M possible code delays and F possible frequency shifts, the acquisition system comprising: a delay line configured to receive and sample said spread spectrum signal to obtain a sampled spread spectrum signal; a vector multiplier configured to despread said sampled spread spectrum signal with a local replica signal of said code signal for a plurality of possible code delays between said sampled signal and said replica signal, said vector multiplier configured to despread using consecutive long segments of the sampled signals, the long segments being of length N, where N is a submultiple integer of a sample number denoted by M of the local replica signal, a number Mn of the despreaded segments of the sampled signals being given by Mn=M/N; and a Fourier transformer configured to compute a Fourier transform, wherein said frequency search is a parallel frequency search performed on said despreaded spread spectrum signal, said Fourier transform being a fractional Fourier transform, wherein the vector multiplier is configured to despread, using each of said Mn segments of the local replica signal, N possible successive delays between said local replica signal and sampled signal.

14. A Global Navigation Satellite System (GNSS) signal receiver comprising: an acquisition system configured to acquire a direct-sequence spread spectrum signal transmitted on a carrier frequency and modulated with a code signal of length equal to Nc chips, to determine a code delay of said spread spectrum signal and a Doppler shift with respect to said carrier frequency, said determination being performed on a discrete two-dimensional space of M possible code delays and F possible frequency shifts, the acquisition system comprising: a delay line configured to receive and sample said spread spectrum signal to obtain a sampled spread spectrum signal; a vector multiplier configured to despread said sampled spread spectrum signal with a local replica signal of said code signal for a plurality of possible code delays between said sampled signal and said replica signal, said vector multiplier configured to despread using consecutive long segments of the sampled signals, the long segments being of length N, where N is a submultiple integer of a sample number denoted by M of the local replica signal, a number Mn of the despreaded segments of the sampled signals being given by Mn=M/N; and a Fourier transformer configured to compute a Fourier transform, wherein said frequency search is a parallel frequency search performed on said despreaded spread spectrum signal, said Fourier transform being a fractional Fourier transform, wherein the vector multiplier is configured to despread, using each of said Mn segments of the local replica signal, N possible successive delays between said local replica signal and sampled signal.